As an example of an element containing an organic compound, a light-emitting element containing a light-emitting organic compound or light-emitting inorganic compound as a light-emitting material has been developed. For example, a light-emitting element called EL (electroluminescent) element having a simple structure in which a light-emitting layer containing a light-emitting material is provided between a pair of electrodes has been studied.
Owing to the simple structure, a thinner and lighter EL element can be easily prepared. Further, the EL element has capabilities for high-speed response with respect to input signals, driving with a DC power at a relatively low voltage (about several volts to several tens of volts), and formation in a large-area film form and thus has been applied to a display device and a lighting device.
In the EL element, from a pair of electrodes between which a light-emitting layer is interposed, electrons and holes are injected to be recombined in the light-emitting layer. Thus, energy is generated and used for exciting a light-emitting substance in the light-emitting layer. The excited light-emitting substance emits light when relaxing to a ground state, whereby the light is extracted and used.
A light-emitting substance can have two types of the excited states: a singlet excited state (S*) and a triplet excited state (T*). The statistical generation ratio thereof in a light emitting element is considered to be S*:T*=1:3.
For a compound that emits light from the singlet excited state (hereinafter, the compound will be referred to as fluorescent compound) at room temperature, only emission from the singlet excited state (fluorescence) is observed, and no emission from the triplet excited state (phosphorescence) is observed. The internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element containing a fluorescent compound is assumed to have a theoretical limit of 25% based on S*:T*=1:3.
On the other hand, the internal quantum efficiency of the EL element utilizing a compound that emits light from the triplet excited state (hereinafter, the compound will be referred to as phosphorescent compound) can be increased to 75% to 100% in theory, considering intersystem crossing from a singlet excited state to a triplet excited state. In other words, emission efficiency can be 3 to 4 times as much as that of the EL element utilizing the fluorescent compound. Therefore, the light-emitting element containing a phosphorescent compound has been actively developed in recent years in order to achieve a high-efficiency light-emitting element. As the phosphorescent compound, an organometallic complex that has iridium or the like as a central metal have particularly attracted attention owing to their high phosphorescence quantum yield; for example, an organometallic complex that has iridium as a central metal is disclosed as a phosphorescent material in Patent Document 1.